Link adaptation method and apparatus in wireless LAN system

ABSTRACT

A link adaptation method performed by a station (STA) in a wireless local area network (LAN) system supporting multi-user multiple-input multiple-output (MU-MIMO) is provided. The method includes: receiving a modulation and coding scheme (MCS) request, a steered sounding physical layer convergence procedure (PLOP) protocol data unit (PPDU) which is beam-formed to the STA, and a MIMO indicator including MU-MIMO-related information from an access point (AP); and transmitting feedback information including an MCS acquired from the steered sounding PPDU and the MU-MIMO-related information to the AP in response to the MCS request.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2010/005464, filed on Aug. 18, 2010,which claims the benefit of U.S. Provisional Application Ser. No.61/295,665, filed on Jan. 15, 2010, the contents of which are allincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a link adaptation method in a wireless local areanetwork (LAN) system and an apparatus supporting the method.

BACKGROUND ART

With the recent development of information communication technology, avariety of wireless communication techniques are being developed. Fromamong them, a Wireless Local Area Network (WLAN) is a technique forwirelessly accessing the Internet at homes or companies or in specificservice providing areas by using portable terminals, such as a PersonalDigital Assistant (PDA), a laptop computer, and a Portable MultimediaPlayer (PMP), based on wireless frequency technology.

A lot of standardization tasks are being performed since Institute ofElectrical and Electronics Engineering (IEEE) 802 (i.e., thestandardization organization of WLAN technology) was established onFebruary, 1980. WLAN technology initially supported a speed of 1 to 2Mbps through frequency hopping, band spreading, and infraredcommunication by using a frequency of 2.4 GHz according to IEEE 802.11,but recently may support a maximum speed of 54 Mbps by using OrthogonalFrequency Division Multiplexing (OFDM). In addition, in IEEE 802.11,standardizations for various techniques, such as the improvement ofQuality of Service (QoS), Access Point (AP) protocol compatibility,security enhancement, radio resource measurement, wireless accessvehicular environments, fast roaming, a mesh network, interworking withan external network, and wireless network management, are being put topractical use or developed. Furthermore, in order to overcome a limit tothe communication speed that was considered as being weakness in theWLAN, IEEE 802.11n has recently been established as a technicalstandard. An object of IEEE 802.11n is to increase the speed andreliability of a network and to extend the coverage of a wirelessnetwork. More particularly, in order to support a High Throughput (HT)having a maximum data processing speed of 540 Mbps or higher, minimizean error in transmission, and optimize the data speed, IEEE 802.11n isbased on Multiple Inputs and Multiple Outputs (MIMO) technology usingmultiple antennas on both sides of a transmitter and a receiver. For anecessity for high quality and broadband data transmission according toincreased users and in order to reduce transmit power, use radioresources efficiently, and extend the service coverage, the IEEE 802.11nstandard supports beamforming technology and data transmission through amaximum of four spatial streams. Furthermore, this standard may use notonly a coding scheme for transmitting several redundant copies in orderto increase data reliability, but also Orthogonal Frequency DivisionMultiplex (OFDM) in order to increase the speed.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. However, an IEEE 802.11n medium accesscontrol (MAC)/physical layer (PHY) protocol is not effective to providea throughput of 1 Gbps or higher. This is because the IEEE 802.11nMAC/PHY protocol is designed for an operation of a single station (STA),that is, an STA having one network interface card (NIC), and thus when aframe throughput is increased while conforming to the conventional IEEE802.11n MAC/PHY protocol, a resultant additional overhead is alsoincreased. Consequently, there is a limitation in increasing athroughput of a wireless communication network while conforming to theconventional IEEE 802.11n MAC/PHY protocol, that is, a single STAarchitecture.

Therefore, to achieve a data processing rate of 1 Gbps or higher in thewireless communication system, a new system different from theconventional IEEE 802.11n MAC/PHY protocol (i.e., the single STAarchitecture) is required. A very high throughput (VHT) WLAN system is anext version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11WLAN systems which have recently been proposed to support a dataprocessing rate of 1 Gbps or higher in a MAC service access point (SAP).

The VHT WLAN system allows simultaneous channel access of a plurality ofVHT non-AP STAs for the effective use of a radio channel. For this,multi-user multiple input multiple output (MU-MIMO)-based transmissionusing multiple antennas is supported. A VHT access point (AP) canconcurrently transmit spatial-multiplexed data to a plurality of VHTnon-AP STAs.

The channel estimation and link adaptation procedure in the conventionalIEEE 802.11n WLAN system specifies a channel estimation and linkadaptation procedure for a plurality of spatial streams between the APand the STA. In comparison thereto, a channel estimation and linkadaptation procedure is required to transmit data as the suitable numberof spatial streams and modulation and coding scheme (MCS) value to eachof a plurality of VHT STAs which are paired as a target of MU-MIMOtransmission in the VHT WLAN system. There is a need to consider a newprotocol for a link adaptation procedure in a WLAN system supportingMU-MIMO.

SUMMARY OF INVENTION Technical Problem

The present invention provides a link adaptation method and apparatusapplicable to a wireless local area network (LAN) system supportingmulti-user multiple input multiple output (MU-MIMO) transmission.

Technical Solution

In an aspect, a link adaptation method performed by a station (STA) in awireless local area network (LAN) system supporting multi-usermultiple-input multiple-output (MU-MIMO) is provided. The methodincludes: receiving a modulation and coding scheme (MCS) request, asteered sounding physical layer convergence procedure (PLOP) protocoldata unit (PPDU) which is beam-formed to the STA, and a MIMO indicatorincluding MU-MIMO-related information from an access point (AP); andtransmitting feedback information including an MCS acquired from thesteered sounding PPDU and the MU-MIMO-related information to the AP inresponse to the MCS request.

The steered sounding PPDU may include a PLOP header, and the steeredsounding PPDU may be indicated by a sounding field in the PLOP header.

The MIMO indicator may indicate the number of paired STAs which aretargets of MU-MIMO transmission together with the STA.

The MIMO indicator may be transmit power information for the STA inMU-MIMO transmission.

The MIMO indicator may further include information regarding the totalnumber of spatial streams used by the AP in MU-MIMO transmission and thenumber of spatial streams assigned to the STA.

The method may further include: receiving from the AP a training requestmessage (TRQ) requesting transmission of a sounding PPDU for channelestimation between the AP and the STA; and transmitting a sounding PPDUin response to the TRQ. The steered sounding PPDU may be beam-formed byusing a precoding matrix determined from the sounding PPDU.

The feedback information may further include information regarding thenumber of spatial streams, determined from the steered sounding PPDU andthe MU-MIMO related information.

The method may further include transmitting a sounding PPDU to the APtogether with the feedback information.

The sounding PPDU may be beam-formed by using a precoding matrixacquired from the steered sounding PPDU.

The method may further include: receiving full-dimension channelinformation between the AP and the STA from the AP; and transmitting anMCS value for a case of assuming single user (SU)-MIMO transmissionbetween the AP and the STA on the basis of the full-dimension channelinformation.

Advantageous Effects

The present invention provides a link adaptation method applicable to awireless local area network (WLAN) system supporting multi-user multipleinput multiple output (MU-MIMO), and allows an optimal modulation andcoding scheme (MCS) to be configured adaptively for a channel situationand transmit power for each of paired stations (STAs) in a MU-MIMOtransmission environment. Accordingly, power consumption can bedecreased, and reliability of data transmission can be increased.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an exemplary structure of a wirelesslocal area network (LAN) system to which an embodiment of the presentinvention can be applied.

FIG. 2 is a message flow chart showing an example of a channelestimation and link adaptation procedure used in multiple input multipleoutput (MU-MIMO) transmission.

FIG. 3 shows an example of a channel estimation method for MU-MIMOtransmission.

FIG. 4 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 5 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 6 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 7 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 8 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 9 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 10 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 11 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

FIG. 12 is a block diagram showing an example of a wireless apparatusaccording to an embodiment of the present invention.

MODE FOR INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. Embodiments of the presentinvention described below can be effectively applied to a very highthroughput (VHT) wireless local area network (WLAN) system supportingmulti-user multiple input multiple output (MU-MIMO). Although the VHTWLAN system will be described hereinafter for example, the technicalfeatures of the present invention are not limited thereto. Thus, thelink adaptation method proposed in the present invention is equallyapplicable to a wireless communication system supporting frametransmission based on a MU-MIMO scheme.

FIG. 1 is a schematic view showing an exemplary structure of a WLANsystem to which an embodiment of the present invention can be applied.

Referring to FIG. 1, the WLAN system includes one or more basis servicesets (BSSs). The BSS is a set of stations (STAs) which are successfullysynchronized to communicate with one another, and is not a conceptindicating a specific region. The BSS can be classified into aninfrastructure BSS and an independent BSS (IBSS). The infrastructure BSSis shown in FIG. 1. Infrastructure BSSs (i.e., BSS1 and BSS2) includeone or more STAs (i.e., STA1, STA3, and STA4), access points (APs) whichare STAs providing a distribution service, and a distribution system(DS) connecting a plurality of APs (i.e., AP1 and AP2). On the otherhand, the IBSS does not include APs, and thus all STAs are mobile STAs.In addition, the IBSS constitutes a self-contained network sinceconnection to the DS is not allowed.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer interface conforming tothe institute of electrical and electronics engineers (IEEE) 802.11standard, and includes both an AP and a non-AP STA in a broad sense. AVHT STA is defined as an STA that supports the super high-speed dataprocessing of 1 GHz or higher in the multi-channel environment to bedescribed below. In the VHT WLAN system to which the embodiment of thepresent invention is applicable, STAs included in the BSS may be all VHTSTAs, or a VHT STA and a legacy STA (i.e., IEEE 802.11n-based HT STA)may coexist.

Among the STAs, non-AP STAs (i.e., STA1, STA3, STA4, STAG, STA7, andSTAs) are portable terminals operated by users. A non-AP STA may besimply referred to as an STA. The non-AP STA may also be referred to asa terminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal, a mobile subscriberunit, etc. A non-AP VHT-STA (or simply VHT STA) is defined as a non-APSTA that supports the super high-speed data processing of 1 GHz orhigher in the multi-channel environment to be described below.

The AP (i.e., AP1 and AP2) is a functional entity for providingconnection to the DS through a wireless medium for an associated STA.Although communication between non-AP STAs in an infrastructure BSSincluding the AP is performed via the AP in principle, the non-AP STAscan perform direct communication when a direct link is set up. Inaddition to the terminology of an access point, the AP may also bereferred to as a centralized controller, a base station (BS), a node-B,a base transceiver system (BTS), a site controller, etc. A VHT AP isdefined as an AP that supports MU-MIMO transmission described below.

A plurality of infrastructure BSSs can be interconnected by the use ofthe DS. An extended service set (ESS) is a plurality of BSSs connectedby the use of the DS. STAs included in the ESS can communicate with oneanother. In the same ESS, a non-AP STA can move from one BSS to anotherBSS while performing seamless communication.

The DS is a mechanism whereby one AP communicates with another AP. Byusing the DS, an AP may transmit a frame for STAs associated with a BSSmanaged by the AP, or transmit a frame when any one of the STAs moves toanother BSS, or transmit a frame to an external network such as a wirednetwork. The DS is not necessarily a network, and has no limitation inits format as long as a specific distribution service specified in theIEEE 802.11 can be provided. For example, the DS may be a wirelessnetwork such as a mesh network, or may be a physical structure forinterconnecting APs.

In the following description of the present invention, transmission ofthe spatially multiplexed data to a plurality of STAs is referred to asSDMA transmission. To perform SDMA transmission by allowing theplurality of STAs to simultaneously access a channel, the STAs maysimultaneously perform transmission through a plurality of spatialstreams by using respective multiple antennas. Multi-user multiple inputmultiple output (MU-MIMO) is a method in which each of a plurality ofSTAs employing multiple antennas simultaneously transmits and/orreceives an independent data stream. Downlink (DL) MU-MIMO implies thatone STA performs MU-MIMO transmission to a plurality of receiving STAs.In this case, one transmitting STA may be an AP, and a plurality ofreceiving STAs may be non-AP STAs. Hereinafter, when a plurality of STAsare paired, it implies that the STAs are paired as target STAs ofMU-MIMO transmission, and paired STAs are STAs paired as transmissiontarget STAs of MU-MIMO.

FIG. 2 is a message flow chart showing an example of a channelestimation and link adaptation procedure used in MIMO transmission.

The example of FIG. 2 is a case in which an AP 200 transmits data to anSTA 260. The AP 200 must know a channel environment with respect to theSTA 260 which intends to transmit data in order to determine amodulation and coding scheme (MCS) and the number of spatial streamssuitable for data transmission. Before data transmission, the channelestimation and link adaptation procedure is required to determine theMCS, the number of spatial streams to be used in data transmission, etc.

In the example of channel estimation and link adaptation of FIG. 2, whenthe AP 200 transmits a training request (TRQ) to the STA (S210), the STAtransmits a sounding PPDU to the AP in response to the TRQ (step S220).The AP estimates channel information on the basis of the receivedsounding PPDU, and transmits data to the STA by determining the MCS andthe number of spatial streams suitable for the channel information (stepS230).

When the STA has capability of sending a feedback to the AP through anMCS selection process, a procedure for determining an MCS value suitablefor data transmission can be performed. When the AP 200 transmits an MCSrequest (MRQ) to the STA 260 together with a sounding PPDU (step S240),the STA 260 selects the suitable number of spatial streams and MCSthrough channel estimation and transmits an MCS response to the AP 200(step S250). In this case, MCS information suitable for a channelsituation is included in the MCS response, and the MCS response is alsocalled an MCS feedback (MFB). The AP 200 may determine the MCS value onthe basis of the MFB received from the STA 260, and may transmit datasubjected to modulation and coding by applying the determined MCS value(step S260). In this case, the sounding PPDU transmitted together withthe MCS request is used so that the AP 200 transmits full-dimensioninformation to the STA 260, and allows the STA 260 to be able totransmit a correct feedback to the AP 200. When the MCS request istransmitted together with the sounding PPDU, the sounding PPDU may betransmitted in a format which includes a field for indicating whetherthe MCS is requested (or indicating that the MCS feedback is requested),or the MCS request and the sounding PPDU may be transmitted throughseparate frames. Hereinafter, unless specified otherwise, theaforementioned two cases are included when it is said that the MCSrequest and the sounding PPDU (or a steered sounding PPDU) aretransmitted together.

FIG. 3 shows an example of a channel estimation method for MU-MIMOtransmission.

In the example of FIG. 3, an AP 300 intends to transmit data to pairedSTAs 360. The AP 300 transmits a TRQ to the paired STAs 360 (step S310).In this case, the TRQ may include a list of paired STAs requiring aresponse (i.e., an STA1 and an STA2 in the example of FIG. 3) and anorder of sounding PPDU responses of the STA1 and the STA2. Uponreceiving the TRQ, the STA1 transmits a sounding PPDU1 to the AP 300(step S320-1), and the STA2 transmits a sounding PPDU2 to the AP 300(step S320-2).

The AP 300 can perform channel estimation on the basis of the receivedsounding PPDU1 and sounding PPDU2. An MCS and/or the number of spatialstreams to be used for data transmission with respect to each of pairedSTAs and suitable for a channel situation can be determined according toa result of channel estimation. The AP 300 transmits data 1 and data 2through MU-MIMO transmission with respect to the paired STAs by usingthe determined MCS and/or the number of spatial streams (step S330). Inthe example of FIG. 3, transmission of the data 1 and transmission ofthe data 2 are depicted by being aggregated with a symbol “⊂”, whichimplies that the data 1 and the data 2 are simultaneously transmitted.In other words, the data 1 and the data 2 are transmitted throughMU-MIMO transmission with respect to the paired STA1 and STA2.Hereinafter, a procedure of transmitting data or messages aggregatedwith the symbol “⊂” also implies a procedure in whichtransmissions/operations are performed simultaneously through MU-MIMOtransmission.

A link adaptation procedure specified in the conventional IEEE 802.11nstandard premises MIMO transmission for a single STA, that is, singleuser (SU)-MIMO transmission. In the conventional link adaptationprocedure, a feedback is performed under the assumption that transmitpower of the AP is entirely used by the single STA in case of SU-MIMO.However, since several STAs can simultaneously receive data from the APin case of MU-MIMO, transmit power that can be used by each of pairedSTAs is decreased to that extent. That is, an MCS and the number ofspatial streams to be used in data transmission for each STA may differaccording to the number of paired STAs. As such, in the link adaptationprocedure for MU-MIMO transmission, there is a need to perform the linkadaptation procedure by considering that a destination STA of datatransmission is not a single STA but a plurality of STAs. That is, thelink adaptation procedure needs to allow each of paired STAs to transmitan MFB to the AP by considering transmit power distributed to each ofpaired STAs.

According to the embodiment of the link adaptation procedure proposed inthe present invention, a MIMO indicator is further transmitted togetherwith the MCS request so that an STA that must transmit the MFB uponreceiving the MCS request can know transmit power distributed to theSTA.

The MIMO indicator according to the embodiment of the present inventionincludes transmit power information for an STA which receives the MCSrequest and/or information regarding the number of STAs paired forMU-MIMO transmission. As an example of the MIMO indicator, a powerindicator (PI) including transmit power information for an STA inMU-MIMO transmission will be described hereinafter. However, the PI isonly one example of the MIMO indicator. Thus, in the following example,the PI may be replaced with information regarding the number of pairedSTAs or may further include information regarding the number of pairedSTAs.

Upon receiving the MCS request, the STA can recognize informationregarding transmit power distributed to the STA in MU-MIMO transmissionon the basis of the PI by receiving the PI together with the MCSrequest, and can use the information to determine an MCS value includedin the MFB. The PI may include information regarding the number of STAspaired for MU-MIMO transmission and/or transmit power information foreach of the paired STAs. In addition, the PI may further includeinformation indicating the total number of spatial streams used by theAP in MU-MIMO transmission and information regarding the number ofspatial streams assigned for each of the paired STAs.

Upon receiving the PI, the STA can know the information regardingtransmit power distributed to the STA or the number of STAs paired forMU-MIMO including the STA. In addition, the STA can know the number ofspatial steams to be used by the AP in MU-MIMO transmission and can alsoknow how many spatial streams are assigned to the STA among the spatialstreams.

Hereinafter, the link adaptation method in which the AP supportingMU-MIMO transmission transmits the PI to the paired STAs and performsthe link adaptation procedure adaptively to a change of a channelenvironment will be described according to detailed embodiments. Thelink adaptation method based on the present invention allows a properfeedback considering a MU-MIMO characteristic in which data issimultaneously transmitted for a plurality of STAs.

FIG. 4 is a message flow chart of a link adaptation procedure accordingto an embodiment of the present invention.

An AP 400 transmits to a plurality of destination STAs 460 which aretargets of MU-MIMO transmission a TRQ and a MU-MIMO primitive forreporting that a procedure for MU-MIMO transmission will start (stepS410). Upon receiving the TRQ, each of paired STAs (i.e., STA 1 and STA2) 460 transmits a sounding PPDU to the AP 400. That is, in the exampleof FIG. 4, the STA 1 transmits a sounding PPDU 1 (step S420-1), and theSTA 2 transmits a sounding PPDU 2 (step S420-2).

For link adaptation, the AP transmits an MRQ 1 and an MRQ 2 to the STA 1and the STA 2, and in this case, a PI 1 and a PI 2 are transmittedtogether, respectively (step S430). The MRQ can be transmitted togetherwith a steered sounding PPDU. In this case, a precoding vectormultiplied by the steered sounding PPDU is a beamforming vector whichforms a beam used by the AP in MU-MIMO transmission. The precodingvector and the beamforming vector are also called a precoding matrix anda beamforming matrix, respectively, and hereinafter will be referred toas the precoding vector and the beamforming vector.

When transmitting the MRQ 1 and the MRQ 2 respectively to the STA 1 andthe STA 2, the AP 400 transmits a steered sounding PPDU which isprecoded with a beamforming vector to be used for beamforming in nextMU-MIMO transmission, so that the paired STAs 460 can estimate a channelsituation in a MU-MIMO transmission environment in which data isactually transmitted and can transmit an MFB by determining an MCS valueon the basis of the estimated channel situation. In other words, thepaired STAs are allowed to determine an MCS feedback by considering aninfluence of a beam formed in the process of performing MU-MIMOtransmission by the AP. The STA can transmit the MFB to another STA byconsidering a beamforming vector to be used in MU-MIMO transmission andacquired by receiving the steered sounding PPDU.

The PI transmitted together with the steered sounding PPDU includesinformation regarding the number of paired STAs or information regardingtransmit power distributed to the STA. The STA can determine the MCS byconsidering the PI, and can transmit the determined MCS to the AP byusing the MFB.

The paired STA 460 transmits the MFB 1 and the MFB 2 to the AP inresponse to the MRQ (steps S440-1 and S440-2). The AP determines the MCSon the basis of the received MFB 1 and MFB 2, and transmits data throughMU-MIMO according to the determined MCS (step S450).

FIG. 5 is a message flow chart of a link adaptation procedure accordingto another embodiment of the present invention.

Similarly to the example of FIG. 4, an AP 500 transmits to a pluralityof destination STAs 560 which are targets of MU-MIMO transmission a TRQand a MU-MIMO primitive for reporting that a procedure for MU-MIMOtransmission will start (step S510). Each of paired STAs 560 whichreceive the TRQ transmits a sounding PPDU to the AP 500. That is, in theexample of FIG. 5, the STA 1 which is a paired STA transmits a soundingPPDU 1 (step S520-1), and the STA 2 transmits a sounding PPDU 2 (stepS520-2).

Upon receiving the sounding PPDU 1 and the sounding PPDU 2 from the STA1 and the STA 2, the AP 500 transmits an MRQ 1 and an MRQ 2 for linkadaptation to the STA 1 and the STA 2. Similarly to the example of FIG.4, the MRQ can be transmitted using a steered sounding PPDU. In thiscase, the PI can be transmitted by being included in the steeredsounding PPDU. That is, unlike the example of FIG. 4, PI information canbe included as a coefficient to a precoding vector multiplied by thesteered sounding PPDU. In other words, the precoding vector multipliedby the steered sounding PPDU may be a matrix obtained by multiplying thePI information as a coefficient by a beamforming vector for beamformingused by the AP in MU-MIMO transmission. The PI can be transmittedindependently from an MRQ transmitted using the steered sounding PPDU asin the example of FIG. 4 or can be transmitted as a part of it by beingincluded in the steered sounding PPDU as in the example of FIG. 5.

Although the embodiment of the present invention described hereinafteris an embodiment in which the PI information is transmittedindependently from the MRQ, this is for exemplary purposes only. Thus,as described in the example of FIG. 5, the PI information can betransmitted by being included in the steered sounding PPDU used in MRQtransmission, which is also applicable to the following descriptions.

Upon receiving the MRQ, the STA 1 receives the steered sounding PPDU 1,acquires PI information and a beamforming vector from a precoding vectorof the steered sounding PPDU 1, determines an MCS by considering the PIinformation and the beamforming vector, and transmits an MFB 1 to the AP500 (step S540-1). Likewise, upon receiving the MRQ, the STA 2 receivesthe steered sounding PPDU 2, acquires PI information and a beamformingvector from a precoding vector of the steered sounding PPDU 2,determines an MCS by considering the PI information and the beamformingvector, and transmits an MFB 2 to the AP 500 (step S540-2). In thiscase, information regarding the number of spatial streams to be used intransmission suitable for a channel situation can be included in the MFB1 and the MFB 2.

The AP 500 determines the MCS and the number of spatial streams on thebasis of the MFB 1 and the MFB 2, and transmits data 1 and data 2 to theSTA 1 and the STA 2 by applying/using the determined MCS and the numberof spatial streams (step S550).

FIG. 6 is a message flow chart of a link adaptation procedure accordingto another embodiment of the present invention.

An example of the link adaptation procedure of FIG. 4 and FIG. 5 is aprotocol including all procedures required when an AP performs linkadaptation on paired STAs. However, if a channel situation does notchange frequently, it may effective to operate the link adaptation byusing a minimum protocol. The link adaptation method of FIG. 6 shows anexample of a link adaptation procedure in which an AP 600 and pairedSTAa 660 transmit data through a link establishment and adaptationprocedure, and can apply it when intended to use the link adaptationprocedure to transmit data at a later time.

After transmitting the data using MU-MIMO transmission through the linkadaptation procedure, the AP 600 transmits again an MRQ to the pairedSTAs 660 to achieve link adaptation for MU-MIMO transmission (stepS610). The AP 600 transmits the MRQ together with a PI under theassumption that a channel environment does not change. The MRQ can betransmitted through a steered sounding PPDU. In this case, a precodingvector used for the steered sounding PPDU is a beamforming vector usedto form a beam for previously transmitted data.

In response to MRQ transmission of step S610, the STA 1 transmits theMFB 1 (step S620-1), and the STA 2 transmits the MFB 2 (step S620-2). Inthis case, the paired STAs (i.e., STA 1 and STA 2) 660 can transmit asounding PPDU together with the MFB 1 and the MFB 2 in order to reportfull-dimension channel information.

The AP 600 can know an extent of channel change by using the soundingPPDU received together with the MFB. If the channel is not much changedfrom previous data transmission, data can be transmitted bydetermining/applying an MCS and the number of spatial streams on thebasis of the MCS and the number of spatial streams of the MFB 1 and MFB2 fed back from the STA 1 and the STA 2 (step S630).

However, if the AP 600 measures a channel of the sounding PPDU receivedtogether with the MFB and the result of measurement shows that a channelenvironment changes in the meantime, the MFB 1 and MFB 2 which aredetermined by receiving a steered sounding PPDU on the basis of thechannel environment in previous data transmission may be not suitablefor the changed channel environment. An embodiment of the linkadaptation procedure applicable to this situation is described belowwith reference to FIG. 7.

FIG. 7 is a message flow chart of a link adaptation procedure accordingto another embodiment of the present invention.

Steps S710, S720-1, and S720-2 of FIG. 7 are the same as steps S610,S620-1, and S620-2 of FIG. 6. However, the embodiment of FIG. 7 shows anexample of a link adaptation procedure in which an AP 700 knows that achannel situation is changed from previous data transmission, by using asounding PPDU 1 and sounding PPDU 2 received together with an MFB 1 andan MFB 2 in steps S720-1 and S720-2.

According to a result of channel estimation using a sounding PPDUreceived together with an MFB from the STA 1 and the STA 2 which areexemplified as paired STAs 760, if the AP 700 knows that a channelsituation is changed from a channel situation of previous datatransmission, the AP 700 transmits again the MRQ and a PI to the STA 1and the STA 2 (step S730). This is because, if the channel situation ischanged, a steered sounding PPDU 1 and steered sounding PPDU 2transmitted to the STA 1 and the STA 2 on the basis of a channelsituation of previous data transmission are no longer suitable for acurrent channel situation, and an MFB 1 and MFB 2 determined based onthe steered sounding PPDU 1 and steered sounding PPDU 2 not suitable forthe current channel situation are not suitable for the current channelsituation, either. Therefore, for new link adaptation, the AP transmitsan MRQ through a steered PPDU 3 and steered PPDU 4 which are re-steeredby considering a result of performing channel estimation by using thesounding PPDU 1 and sounding PPDU 2 received in steps S720-1 and S720-2,and also transmits a PI 3 and PI 4 re-steered in the same manner (stepS730). In this case, a precoding vector used in the steered PPDU 3 andthe steered PPDU 4 is a transmission (Tx) vector of a channel measuredby using the sounding PPDU 1 and sounding PPDU 2 transmitted in stepsS730-1 and S730-2.

The STA 1 transmits an MFB 3 and a sounding PPDU 3 to the AP in responseto an MRQ (i.e., a steered sounding PPDU 3) (step S740-1), and the STA 2transmits an MFB 4 and a sounding PPDU 4 to the AP in response to an MRQ(i.e., a steered sounding PPDU 4) (step S740-2).

If it is determined that a channel is not much changed as a result ofchannel estimation based on the sounding PPDU 3 and sounding PPDU 4received in steps S 740-1 and S740-2, the AP determines an MCS accordingto the MFB 3 and MFB 4 received in step S 740-1 and S740-2 and transmitsdata 1 and data 2 (step S750).

The link adaptation procedure shown in the example of FIG. 7 iseffectively applicable to any one transmission opportunity (TXOP)duration. For one example, in case of a TXOP holder in which the STA1and STA 2 of FIG. 7 obtain the TXOP, other STAs except for the STA 1 andthe STA 2 defer medium access during the TXOP duration. Therefore, whilerepetitively operating data transmission and the link adaptationprocedure of FIG. 7 between the STA 1 and the STA 2 exemplified as thepaired STAs 760 during the TXOP duration, the AP can effectively supportMU-MIMO link adaptation.

FIG. 8 is a message flow chart of a link adaptation procedure accordingto another embodiment of the present invention.

An example of FIG. 8 is an example of a link adaptation procedure when apersistent link adaptation is not used. Unlike FIG. 6, each of MU-MIMOpaired STAs (i.e., STA 1 and STA 2) 860 can transmit a steered soundingPPDU 1 and a steered sounding PPDU 2 to an AP while transmitting an MFB1 and an MFB 2 to the AP. This is because, if the number of transmit(Tx) antennas of the AP is greater than the number of receive (Rx)antennas of all STAs, the AP cannot properly form a beam since a nullspace is not sufficient between the AP and the STA.

FIG. 9 is a message flow chart of a link adaptation procedure accordingto another embodiment of the present invention.

Referring to FIG. 9, an AP can transmit a MU-MIMO primitive, an MRQ, anda PI to an STA 1 and an STA 2 (step S910). In this case, the MU-MIMOprimitive is transmitted in a format that can be recognized by all STAsin a BSS. The MU-MIMO primitive may include information regarding aMU-MIMO transmission time, a transmission duration, etc. HT STAs andlegacy STAs not supporting MU-MIMO can know that MU-MIMO transmissionwill be achieved though the MU-MIMO primitive, and can defer channelaccess by configuring a network allocation vector (NAV) while MU-MIMOtransmission is performed.

When channel access of the STA is regulated by a hybrid coordinationfunction (HCF) of the IEEE 802.11 standard, TXOP can be configured in anenhanced distributed channel access (EDCA) which is a contention-basedchannel access method of the HCF or a contention-free-based HCFcontrolled channel access (HCCA) method. When the TXOP is assigned to acertain STA, only an STA (i.e., a TXOP holder) to which the TXOP isassigned during a corresponding TXOP duration can transmit/receive databy accessing a channel. During the TXOP duration, not only legacy STAsbut also STAs irrelevant to a MU-MIMO operation defer channel access byconfiguring an NAV. In the embodiment of the present inventionexemplified in FIG. 9, a TXOP configuration based on the HCF can also beused. When the STA 1 and the STA 2 obtain TXOP (e.g., EDCA TXOP, HCCATXOP, polled TXOP) and thus become TXOP holders, the remaining STAsexcept for the STA 1 and the STA 2 configure an NAV during a TXOPduration, and defer channel access.

An MRQ 1 which is transmitted simultaneously with a MU-MIMO primitive istransmitted together with a steered sounding PPDU. In this case, thesteered sounding PPDU is a value multiplied by a beam vector/matrix inprevious TXOP.

The STA 1 receives a steered sounding PPDU 1 and a PI 1, determines anMCS value based on this, and transmits the determined MCS value to theAP (step S920-1). The MFB 1 may further include information regardingthe number of spatial streams suitable for a channel situation. In thiscase, a sounding PPDU or a steered sounding PPDU is transmitted togetherwith the MFB 1. The AP can know a change of the channel situation byusing the sounding PPDU or the steered sounding PPDU. The AP determinesan MCS value of data to be transmitted to the STA 1 by using thereceived MFB 1, and transmits data 1 by applying the determined MCS. Theaforementioned link adaptation procedure of the AP and the STA 1 is alsoequally applicable between the AP and the STA 2.

In the aforementioned link adaptation procedure according to theembodiment of the present invention, the AP transmits a sounding PPDU toMU-MIMO paired STAs and thus reports full-dimension channel informationincluding channel information for all spatial streams used by the AP inMU-MIMO transmission. Each of the paired STAs which receive the soundingPPDU can feed back the suitable number of spatial streams and MCSinformation on the basis of the full-dimension channel information.

In this case, in order to provide the full-dimension channelinformation, an extended long training field (LTF) must be transmittedto each STA. For example, it is assumed that the AP performs MU-MIMOtransmission to the STA 1 and the STA 2 by using four spatial streams,and among the four spatial streams, uses two spatial streams intransmission to the STA 1 and the remaining two spatial streams intransmission to the STA 2. In this case, as a result of beamforming forMU-MIMO transmission to the STA 1 and the STA 2, the STA 1 and the STA 2may not be able to recognize data LTFs of the counterpart STA, or maynot be able to receive the data LTFs due to severe interference. In thiscase, the AP can additionally transmit the extended LTF so that each ofthe paired STAs can obtain the full-dimension channel information.

As an example of transmitting the full-dimension channel information bythe AP to each of the paired STAs, a null data packet (NDP) specified inthe IEEE 802.11n Std. and not including a data field can be used. The APcan additionally transmit an NDP frame including full channelinformation while transmitting an MRQ and a PI to the STA. In this case,information indicating that the extended LTF is additionally transmittedcan be transmitted by containing the information in an SIG field of aPLOP header of the NDP frame.

It has been described up to now that, as one of target STAs of MU-MIMOtransmission, the STA determines an MCS value and transmits an MFB tothe AP under the assumption that data is received through MU-MIMOtransmission. According to another embodiment of the present invention,the STA may transmit to the AP an MFB further including an MCS valuewhen the AP performs SU-MIMO transmission for the AP itself, in additionto the MCS value used when MU-MIMO transmission is assumed. In otherwords, an MCS value suitable for a case of using MU-MIMO transmission ina current channel situation and an MCS value suitable for a case ofusing SU-MIMO transmission can be transmitted to the AP. By comparing acase of assuming MU-MIMO transmission with a case of assuming SU-MIMOtransmission, the AP can determine an MCS and a spatial stream of datato be transmitted to the STA.

FIG. 10 is a message flow chart of the aforementioned link adaptationprocedure according to an embodiment of the present invention.

An AP 1000 transmits a MU-MIMO primitive and a TRQ to STAs (i.e., STA 1and STA 2) 1060 (step S1010). The STA 1 and the STA 2 respectivelytransmit a sounding PPDU 1 and a sounding PPDU 2 to the AP 1000 inresponse to the TRQ.

The AP 1000 transmits an MRQ and a PI 1 to the STA 1 and transmits a PI2 to the STA 2 through MU-MIMO transmission (step S1030). In this case,since an MRQ 1 and an MRQ 2 are transmitted through a steered soundingPPDU, the STA 1 and STA 2 which receive the MRQ 1 and the MRQ 2 cannotknow full-dimension channel information, and thus cannot feed back anMCS in an SU-MIMO environment. An NDP frame is additionally transmittedto report full channel information to the STA. An NDP 1 and an NDP 2 mayinclude channel information of all links, or may include information ofthe remaining links which are not delivered through the steered soundingPPDU.

The STA 1 can determine the number of spatial streams and an MCSsuitable for MU-MIMO transmission through the steered sounding PPDU 1and the PI 1, and can acquire full-dimension channel information byusing the NDP 1 (or the NDP and the steered sounding PPDU 1) anddetermine an MCS suitable for SU-MIMO transmission. Likewise, the STA 2can determine the number of spatial streams and an MCS suitable forMU-MIMO transmission through the steered sounding PPDU 2 and the PI 2,and can acquire full-dimension channel information by using the NDP 2(or the NDP and the steered sounding PPDU 1) and determine an MCSsuitable for SU-MIMO transmission.

The STA 1 transmits to the AP 1000 an MCS feedback value for a case ofassuming SU-MIMO transmission and a case of assuming MU-MIMOtransmission by using an MFB 1 (step S1040-1). Likewise, the STA 2transmits to the AP 1000 an MCS feedback value for a case of assumingSU-MIMO transmission and a case of assuming MU-MIMO transmission byusing an MFB 2 (step S1040-2).

The AP 1000 compares a throughput for a case of using SU-MIMOtransmission and a case of using MU-MIMO transmission by using the MFB 1and the MFB 2. If the comparison result shows that a throughput that canbe obtained for the case of using MU-MIMO transmission is greater than athroughput that can be obtained for the case of using SU-MIMOtransmission, data can be transmitted through MU-MIMO transmission, andotherwise data can be transmitted through SU-MIMO transmission. It isassumed that, in Equation 1 to Equation 2, MU_Tput(STA 1+STA 2) denotesa throughput that can be obtained for a case of performing MU-MIMOtransmission to the STA 1 and the STA 2 and SU_Tput(STA X) denotes athroughput that can be obtained for a case of performing SU-MIMOtransmission to an STA X. Then, if a condition of Equation 1 issatisfied, the AP may perform MU-MIMO transmission to the STA 1 and theSTA 2. Further, if a condition of Equation 2 is satisfied, the AP mayperform SU-MIMO transmission to the STA 2. Further, if a condition ofEquation 3 is satisfied, the AP may perform SU-MIMO transmission to theSTA 3.MU _(—) T _(put)(STA1+STA2)≧max(SU _(—) T _(put)(STA1),SU _(—) T_(put)(STA2))  [Equation 1]MU _(—) T _(put)(STA1+STA2)<max(SU _(—) T _(put)(STA1),SU _(—) T_(put)(STA2))

,SU _(—) T _(put)(STA1)≦SU _(—) T _(put)(STA2)  [Equation 2]MU _(—) T _(put)(STA1+STA2)<max(SU _(—) T _(put)(STA1),SU _(—) T_(put)(STA2))

,SU _(—) T _(put)(STA1)>SU _(—) T _(put)(STA2)  [Equation 3]

In the example of FIG. 10, a case of satisfying the condition ofEquation 1 is exemplified. That is, the AP 1000 transmits the data 1 andthe data 2 to the STA 1 and the STA 2 through MU-MIMO transmission.

FIG. 11 is a message flow chart of a link adaptation procedure accordingto another embodiment of the present invention.

An example of FIG. 11 is a case in which transmission can be performedusing full-dimension channel sounding when an MRQ 1 and an MRQ 2 aretransmitted respectively to an STA 1 and an STA 2. That is, sincefull-dimension channel information is transmitted to the STA 1 and theSTA 2 through a sounding PPDU 1 and a sounding PPDU 2 in step S1120,unlike the case of FIG. 10, an NDP may not be additionally transmitted.The remaining operations are the same as in the example of FIG. 10.

FIG. 12 is a block diagram showing an example of a wireless apparatusaccording to an embodiment of the present invention. A wirelessapparatus 1200 may be an AP or a non-AP STA.

The wireless apparatus 1200 includes a processor 1210, a memory 1220,and an RF unit 1230. The RF unit 1230 can transmit/receive a radiosignal, and can perform MIMO transmission or reception through multipleantennas. The processor 1210 is coupled to the RF unit 1230, andimplements a MAC layer and PHY layer of IEEE 802.11. When the processor1210 handles an operation of the AP among the aforementioned methods,the wireless apparatus 1200 is the AP. When the processor 1210 handlesan operation of the STA among the aforementioned methods, the wirelessapparatus 1200 is the STA.

The MAC layer of the wireless apparatus implemented by the processor1210 supports a link adaptation method based on the aforementioned linkadaptation protocol, generates a management frame required for implementthe aforementioned link adaptation method, and transmits the generatedframe to the RF unit 1230 via a PLOP layer and a PMD layer. Each of theMAC layer and PHY layer supporting the link adaptation method accordingto the present invention can be implemented by the processor in a moduleform.

The processor 1210 and/or the RF unit 1230 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. The memory 1220 mayinclude a read-only memory (ROM), a random access memory (RAM), a flashmemory, a memory card, a storage medium, and/or other equivalent storagedevices. When the embodiment of the present invention is implemented insoftware, the aforementioned methods can be implemented with a module(i.e., process, function, etc.) for performing the aforementionedfunctions. The module may be stored in the memory 1220 and may beperformed by the processor 1210. The memory 1220 may be located insideor outside the processor 1210, and may be coupled to the processor 1210by using various well-known means.

Various modifications may be made in the aforementioned embodiments.Although all possible combinations of the various modifications of theembodiments cannot be described, those ordinary skilled in that art willunderstand possibility of other combinations. Therefore, the presentinvention is not intended to be limited to the embodiments shown hereinbut is to be accorded the widest scope consistent with the principlesand novel features disclosed herein.

The invention claimed is:
 1. A method for a wireless local area network (LAN) system supporting multi-user multiple-input multiple-output (MU-MIMO), the method comprising: receiving, by a station (STA), a training request to request a channel feedback from an access point, the training request including a MIMO indicator for indicating paired STAs which are targets of a MU-MIMO scheme together with the STA and which are expected to prepare the requested channel feedback; receiving, by the STA, a null data packet (NDP) from the access point after receiving the training request; and transmitting, by the STA, feedback information to the access point, the feedback information including channel information and stream information, the channel information indicating an estimated channel obtained by assuming the MU-MIMO scheme, and the stream information indicating a number of spatial streams obtained from the NDP.
 2. The method of claim 1, wherein the MIMO indicator indicates a number of the paired STAs which are the targets of the MU-MIMO scheme together with the STA.
 3. The method of claim 1, wherein the NDP includes a long training field and does not include a data field.
 4. The method of claim 1, wherein the training request includes number information indicating a first number, and the estimated channel is measured based on the first number.
 5. A station (STA) for a wireless local area network (LAN) system supporting multi-user multiple-input multiple-output (MU-MIMO), the STA comprising: a radio frequency (RF) unit; and a processor operatively coupled to the RF unit and configured to: instruct the RF unit to receive a training request to request a channel feedback from an access point, the training request including a MIMO indicator for indicating paired STAs which are targets of a MU-MIMO scheme together with the STA and are expected to prepare the requested channel feedback; instruct the RF unit to receive a null data packet (NDP) from the access point after receiving the training request; and instruct the RF unit to transmit feedback information to the access point, the feedback information including channel information and stream information, the channel information indicating an estimated channel measured by assuming the MU-MIMO scheme, the stream information indicating a number of spatial streams obtained from the NDP. 